This invention was made in the course of, or under, a contract with the Energy Research and Development Administration. It relates generally to a novel iridium base alloy composition, and particularly to an alloy suited for use as an encapsulation material for radioisotope fuels. Radioisotope fuels have found considerable use as both terrestrial and space power sources. Such fuels utilize an isotope which is an alpha, beta, or gamma emitter. Heat is produced from these nuclear emissions and converted into electrical energy by means of thermoelectric generators or thermionic converters.
The most prominent radioisotope fuels at present are .sup.238 PuO.sub.2 and .sup.244 Cm.sub.2 O.sub.3. These particular isotopes in the oxide form are desirable because of their refractory properties. The .sup.238 PuO.sub.2 and .sup.244 Cm.sub.2 O.sub.3 are generally sintered into spherical balls or cylindrical pellets.
Radioisotopic fuels which are used in space power systems must be encapsulated in a highly reliable material, not only to contain the fuel for normal operation of several years, but to survive launch abort situations, severe aerodynamic heating on re-entry, high velocity impact, and post-impact oxidizing environment. Various alloys have been developed for use as an encapsulation material in this type of environment. These alloys possess many of the desirable characteristics which are needed for such an encapsulation. Prior to the invention herein disclosed, iridium with a small amount of tungsten was the most prominent of the encapsulation alloys.
An iridium alloy containing about 0.3 wt. percent tungsten is currently used as an encapsulation material for a multihundred watt heat source because of its adequate fabricability, fuel compatability and oxidation resistance. This material, however, exhibits only marginal performance in terms of impact resistance under re-entry conditions; i.e., 1400.degree.C at a velocity of about 300 feet per second.